Vehicle
1996 Ford Thunderbird LX 3.8L
It has the Teves MK IV ABS system with
traction control.
Symptoms
I've been having a problem with my
brakes for a few years now. The rear brakes don't have enough braking
power. When I took my car in for safety inspection, the brake bias
was way too far forward, meaning the fronts did most of the braking,
and the rears barely did anything.
The other problem was that if I pressed
hard on the brake pedal, I could get it to go all the way to the
floor. If I did a panic stop, I could almost get the wheels to lock
up on dry pavement, but this would require lots of pressure on the
brake pedal. I was worried that I would have to press too hard in an
emergency situation.
Attempt 1
So I replaced the master cylinder and
bled the brake system. That didn't help at all. I was considering
replacing the proportioning valve, but didn't because I suspected air
in the ABS unit because of the excessive pedal travel.
I activated the traction control while
bleeding the brakes. I activated the ABS in the snow, then bled the
brakes. I bled the brakes again. Nothing helped.
Research
So I turned to the internet and the
service manual. Both sources said that I needed to get my hands on a
T90P-50-ALA tool. I didn't have the $300 that they wanted for one on
ebay. So I found some wiring diagrams and a pin-out diagram. These
were really helpful, but I still made a couple mistakes along the
way. Hopefully you don't repeat my mistakes.
Attempt 2
This was a near disaster. I blew a fuse
trying to hook up power to the ABS valves. I was connecting to the
wrong end of connector C-104. Basically, I connected power to pins 3
and 33 of the ABS module without the ABS relay being activated. When
the main ABS relay is not activated, it connects pins 3 and 33 to
ground. Connecting power to ground is not a good thing. Something had to melt. It's a good thing there was a fuse between the battery and my connection to ground.
After recovering from that mistake, I
thought you run the pump with all the valves activated. Well, that
made the ABS motor make an awful sound. I hope I didn't damage it.
After putting it all back together, it still wasn't fixed. I was not having a successful day.
The ABS module is located in front of the front wheel on the driver's side. Here's a picture with the inner fender removed and the wiring harness detached.
More Research
Then I took a look at the hydraulic
diagram. I realized my mistake. By closing the inlet valves, I
blocked the path of the brake fluid from the pump, preventing it from circulating. (See dead head definition.)
If I activated only the outlet valve, then the motor could circulate
brake fluid, leaving the air bubbles in the reservoir. The inlet
valves are normally open, while the outlet valves are normally
closed. When you activate either valve, you change the state (open to
closed, or vice-versa). Activating the valve is done by grounding the
appropriate pin on the connector to the ABS module.
This is an ABS diagram similar to my Thunderbird, but it doesn't have the traction control valves in the diagram.
Attempt 3
Here's the approximate procedure I used
to bleed the ABS unit on my Thunderbird without the help of the
T90P-50-ALA tool.
- Remove the front driver's side wheel, remembering to implement all safety precautions.
- Remove the inner fender.
- Disconnect the wiring harness from the ABS control module. You pull on the metal clip and rotate it up to the top. Then lift the connector to remove it from the module.
- Ground pin 34 to activate the main relay. This makes power available to the valves.
- Ground pin 15 for twenty seconds to run the ABS motor.
- Ground pins 2, 21, 36, and 18 to activate each wheel's outlet valves, one at a time while the ABS motor is running. Don't run the motor for more than 60 seconds. Maybe run each valve for 5 seconds each.
- I'm not sure if you're done at this point if you have traction control. But here's what I did next.
- I hadn't found a hydraulic diagram for the traction control part yet, so I opened both rear dump valves (pins 18 and 36) with the ABS motor running while I activated pins 37 and 40, one at a time to run fluid through the traction control part.
I was really hoping this worked.
Unfortunately, it is mostly the same. But it was enough to get it to barely pass the brake skid-pad test at inspection.
More Research
I
found this diagram on the internet. It looks like this is the
traction control version for FWD (still not my RWD version, but maybe close enough to figure it out).
Only the outlet valves are normally
closed. The inlet valves and the two special valves for the traction
control are normally open.
When the traction control is activated,
the special valves close to prevent the ABS unit from applying pressure to the front brakes.
I'll probably have to work on this again to pass the next annual inspection. So here are the next two possible solutions I'm going to try.
Bleed one-way valves
Activate both
traction control valves and dump valves for the back wheels while
pressing on the brake pedal to bleed through the one-way valves.
Close the inlet valve
and compress the caliper piston to bleed each one-way valve on the inlet valves. Do
this for each wheel.
Bleed pressure relief valves
Activate both
traction control valves and the ABS pump.
Hopefully this fixes it. I don't see a way for the $300 bleeder tool to bleed the one-way valves if there's any air stuck in them. So I don't see a point in spending the extra money if there's a chance it won't solve my problem. Inspection is coming up, so if the brakes don't pass this month, I'll be updating this post soon.